A scene can be written in two different ways, with identical dialogue, but with a different point of view. For example:
CARRIE frowns.
merely shows us Carrie's facial expression. The emotional POV is either neutral, or it belongs to some other character who's seeing it. But this:
CARRIE frowns, troubled.
gets us into Carrie's heart just enough for us to feel her emotions, without going so deep into her mind that it can't be filmed.
Let's suppose you have a scene where Carrie walks into a room, overhearing the conversation before she comes in. If you're writing the scene from her POV, then you start the scene with her outside the room, overhearing the conversation. If you're writing from the POV of someone inside the room, then you start the scene in the room, and have her walk in.
INT. OLD MAN'S HOUSE -- CARRIE'S ROOM -- DAY
Carrie wakes up, alert.
SARA (O.C.)
I just read this Wired piece you
wrote?
NICK (O.C.)
Oh yeah? Damn, I must've left it
lying around.
Carrie sits up, eager.
INT. OLD MAN'S LIVING ROOM -- DAY
Carrie pokes her head in:
Sara is unfolding a piece of paper out of her back
pocket. Nick is doing the crossword puzzle on the coffee
table.
INT. OLD MAN'S LIVING ROOM - LATE AFTERNOON
Sara comes in. Nick is doing the crossword puzzle on the
coffee table. She pulls a folded-up piece of paper out
of her back pocket.
SARA
I just read this Wired piece you
wrote?
NICK
Oh yeah? Damn, I must've left it
lying around.
He looks up. Carrie's watching them from the doorway.
In both versions, the same scene played out; in a stage play, you wouldn't be able to tell the difference between the two. In each scene, Carrie overheard the conversation. But in the first version, we're identifying with Carrie, and in the second, we're not. That's because we saw the first version through Carrie's point of view, but in the second, we're in Sara's point of view, and Carrie's overhearing the conversation is a surprise.
The rest of the dialogue in the scene might be exactly the same in both versions. Once you establish the scene's point of view, the scene will stay in that point of view until you do something to change it...